The present invention relates to non-magnetic composite particles, a process for producing the non-magnetic composite particles, and a magnetic recording medium using the non-magnetic composite particles. More particularly, the present invention relates to non-magnetic composite particles having a high polishing effect and a high dispersibility, a magnetic recording medium which is provided with a non-magnetic undercoat layer using the above non-magnetic composite particles, and has an excellent durability and a sufficient surface smoothness, a non-magnetic substrate containing the non-magnetic composite particles, and a process for producing the non-magnetic composite particles.
With the recent tendency toward miniaturization and weight-reduction of video or audio magnetic recording and reproducing apparatuses as well as prolonged recording time of these apparatuses, magnetic recording media such as magnetic tapes or magnetic discs have been strongly required to have a high performance, namely, high recording density, high output characteristics such as, especially, improved frequency characteristics, low noise level or the like.
In particular, recent video tapes have been also required to exhibit higher picture quality, so that the frequencies of carrier signals recorded thereon are shifted to shorter wavelength region (short wave-recording) as compared to those used for conventional video tapes. As a result, the magnetization depth from the surface of the magnetic tape has become remarkably shallow.
In order to improve the high output characteristics, especially, the S/N ratio of a magnetic recording medium when short-wavelength signals are recorded thereon, it has been strongly required to reduce the thickness of a magnetic recording layer of the magnetic recording medium. In order to reduce the thickness of the magnetic recording layer, it is necessary to smoothen the surface of the magnetic recording layer and lessen the un-uniformity of thickness thereof. To meet these requirements, it is also required to smoothen the surface of a base film used in the magnetic recording medium.
With the recent tendency toward reduction in thickness of the magnetic recording layer, in order to solve conventional problems such as poor surface properties and deteriorated electromagnetic performance of the magnetic recording layer, there has been proposed and put into practice a method of providing at least one undercoat layer comprising a binder resin and non-magnetic particles dispersed therein (hereinafter referred to merely as xe2x80x9cnon-magnetic undercoat layerxe2x80x9d) on a non-magnetic base film (Japanese Patent Publication (KOKOKU) No. 6-93297(1994), and Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338(1987), 63-187418(1988), 4-167225(1992), 4-325915(1992), 5-73882(1993) and 5-182177(1993)).
However, these non-magnetic undercoat layers have been strongly required to have further improved surface smoothness. For this reason, it has been attempted to improve the dispersibility of acicular hematite particles used as non-magnetic particles in the non-magnetic undercoat layer.
In addition, the reduced thickness of the magnetic recording layer causes deterioration in durability of the magnetic recording medium itself. Therefore, it has also been strongly required to enhance the durability of the magnetic recording medium.
In order to enhance the durability of the magnetic recording medium itself, abrasives, e.g., oxide particles such as alumina, are added to the magnetic layer or non-magnetic undercoat layer thereof. However, the addition of these abrasives causes many problems. For instance, it is known that alumina has a poor dispersibility in binder resins. Therefore, when a large amount of alumina is added to these layers, there arise defects such as increased dropouts and deteriorated surface smoothness of the obtained magnetic recording medium. In consequence, it has been demanded to provide a non-magnetic undercoat layer and non-magnetic particles used therefor which are capable of imparting a sufficient durability to the obtained magnetic recording medium even when the magnetic recording layer has a small thickness and the amount of abrasives added such as alumina is reduced.
Conventionally, in order to improve various properties of non-magnetic particles, there are known non-magnetic particles having a surface coat composed of an Si compound or an Al compound (Japanese Patent Application Laid-Open (KOKAI) Nos. 5-182177(1993), 5-347017(1993), 6-60362(1994), 10-21532(1998), 10-320753(1993), etc.), or non-magnetic particles on the surface of which fine particles of an Al compound or an Si compound are adhered (Japanese Patent Application Laid-Open (KOKAI) No. 7-192248(1995), etc.).
The non-magnetic particles having a surface coat composed of an Si compound or an Al compound as described in Japanese Patent Application Laid-Open (KOKAI) Nos. 5-182177(1993), 5-347017(1993), 6-60362(1994), 10-21532(1998) and 10-320753(1993) exhibit an improved dispersibility. However, when these non-magnetic particles are used for a non-magnetic undercoat layer, the obtained magnetic recording medium has an insufficient durability. Therefore, the use of these non-magnetic particles cannot reduce the content of abrasives in the magnetic recording medium.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 7-192248(1995), there is described a method of precipitating fine particles of an oxide or hydroxide of Al or Si on the surface of each non-magnetic particle and then fixing the fine particles thereon by compaction and pulverization treatments. However, as shown in Comparative Examples hereinafter, a considerable amount of the fine particles are desorbed or fallen-off from the surface of each non-magnetic particle. Therefore, when the non-magnetic particles are used for a non-magnetic undercoat layer of a magnetic recording medium, the dispersibility thereof is unsatisfactory, so that the obtained magnetic recording medium cannot show a sufficient durability. Accordingly, the use of such non-magnetic particles cannot reduce the content of abrasives in the magnetic recording medium.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that by mixing non-magnetic particles having an average particle size of 0.01 to 0.3 xcexcm with inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm; adhering the inorganic fine particles onto the surface of each non-magnetic particle; adding tetraalkoxysilanes to the obtained particles; and then heating the resultant mixture at a temperature of 40 to 200xc2x0 C. to fix or anchor the inorganic fine particles onto the surface of each magnetic particle through a silicon compound derived from the tetraalkoxysilanes,
the thus obtained non-magnetic composite particles are free from desorption or falling-off of the inorganic fine particles from the surface of each non-magnetic particle, and as a result, can show an excellent dispersibility and a high polishing effect. The present invention has been attained on the basis of this finding.
It is an object of the present invention to provide non-magnetic composite particles for a non-magnetic undercoat layer of a magnetic recording medium, which can exhibit not only an excellent dispersibility but also an excellent polishing effect by firmly fixing or anchoring inorganic fine particles on the surface of each non-magnetic particle.
It is another object of the present invention to provide a non-magnetic substrate for a high-density magnetic recording medium which is excellent in durability and surface smoothness.
It is still another object of the present invention to provide a high-density magnetic recording medium exhibiting excellent durability and surface smoothness.
To accomplish the aims, in a first aspect of the present invention, there are provided non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of each non-magnetic particle, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a second aspect of the present invention, there are provided non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm;
(a1) an undercoat formed on the surface of each non-magnetic particle as core particle and comprising at least one compound selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of the undercoat, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a third aspect of the present invention, there is provided a non-magnetic substrate for magnetic recording medium, comprising:
(1) a base film; and
(2) a non-magnetic undercoat layer formed on the base film, comprising a binder resin and non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of each non-magnetic particle, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a fourth aspect of the present invention, there is provided a non-magnetic substrate for magnetic recording medium, comprising:
(1) a base film; and
(2) a non-magnetic undercoat layer formed on the base film, comprising a binder resin and non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm;
(a1) an undercoat formed on the surface of each non-magnetic particle as core particle and comprising at least one compound selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of the undercoat, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a fifth aspect of the present invention, there is provided a magnetic recording medium comprising:
(1) a non-magnetic base film;
(2) a non-magnetic undercoat layer formed on the base film, comprising a binder resin and non-magnetic composite particles; and
(3) a magnetic recording layer formed on the non-magnetic undercoat layer, comprising magnetic particles and a binder resin,
the said non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of each non-magnetic particle, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a sixth aspect of the present invention, there is provided a magnetic recording medium comprising:
(1) a non-magnetic base film;
(2) a non-magnetic undercoat layer formed on the base film, comprising a binder resin and non-magnetic composite particles; and
(3) a magnetic recording layer formed on the non-magnetic undercoat layer, comprising magnetic particles and a binder resin,
the said non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm;
(a1) an undercoat formed on the surface of each non-magnetic particle as core particle and comprising at least one compound selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of the undercoat, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a seventh aspect of the present invention, there is provided a process for producing the non-magnetic composite particles as defined in the first aspect, comprising:
(i) mixing non-magnetic particles having an average particle size of 0.01 to 0.3 xcexcm with inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, and comprising at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element, to adhere the inorganic fine particles onto the surface of each non-magnetic particle;
(ii) adding tetraalkoxysilanes to the resultant particles; and
(iii) heating the obtained mixture at a temperature of 40 to 200xc2x0 C., thereby fixing or anchoring the inorganic fine particles onto the surface of each non-magnetic particle through a silicon compound derived from the tetraalkoxysilanes.